AC servomotors are becoming used for middle-sized injection molding machines heretofore driven by hydraulic actuators (clamping force>3.5 MN) that have high precision, quick response and higher power which are obtained by performance improvements of permanent magnets and cost reductions.
An injection molding machine consists of a plasticizer in which resin pellets are melted by friction heat generated by plasticizing screw revolution and stored at the end of a barrel, an injector in which an amount of melted polymer is injected into a metal mold at a given velocity and a given dwell pressure is applied, and a clamper in which the metal mold is clamped and opened, all using AC servomotors drive system. FIG. 2 is a view which shows an existing plasticizing mechanism by AC servomotors.
On an injection machine base which is fixed on the ground, a movable base is located which moves on a linear slider and both the bases are not shown in FIG. 2. All parts except a metal mold 1 shown in FIG. 2 are mounted on the movable base. By sliding the movable base, the top of a barrel 2 is clamped on the metal mold 1 and vice versa the top of the barrel 2 is separated from the metal mold 1. FIG. 2 shows a mode in which the resin pellets are melted by the screw revolution in the plasticizing process.
On the movable base, a barrel 2, a servomotor for injection 3, a reduction gear 4, a ball screw 5, a bearing 6 and a hopper 16 are fixed. A nut 7 of the ball screw 5, a moving part 8, a screw 9, a reduction gear 10, a servomotor for plasticization 11 and a pressure detector 12 such as a load cell consist of an integral structure. The moving part 8 is mounted on a linear slider 13 so that the integral structure is moved back and forth by the movement of the nut 7.
Rotation of the servomotor for injection 3 is transferred to the ball screw 5 which magnifies a linear force through the reduction gear 4 and the rotation of the ball screw 5 is converted to a linear motion of the nut 7 of the ball screw 5 and through the moving part 8, a linear motion of the screw 9 and pressure application to the stored melted polymer are realized. Pressure applied to the melted polymer by the screw 9 in the plasticizing process is hereinafter referred to as a screw back pressure. Position of the screw 9 is detected by a rotary encoder 14 mounted on the servomotor for injection 3. Screw back pressure to the melted polymer stored at the end of the barrel 2 is detected by the pressure detector 12 such as a load cell mounted between the nut 7 and the moving part 8. The screw 9 is rotated by the servomotor for plasticization 11 through the reduction gear 10 in the plasticizing process in which resin pellets are melted and kneaded and a rotary encoder 15 is mounted on the servomotor for plasticization 11.
Explaining an injection molding process with referent to FIG. 2, resin pellets are fed to the screw 9 through the hopper 16 and are melted by the screw 9 rotated by the servomotor for plasticization 11 and the melted polymer is pushed out from the top of the screw 9 and the screw 9 is moved back by the generated screw back pressure. The screw back pressure is a linear force applied to the melted polymer decided by a generated motor torque of the servomotor for injection 3. The servomotor for plasticization 11 continues to rotate until a given amount of melted polymer necessary for molding a product is stored at the end of the barrel 2 and then the plasticizing process is finished with the stop of the screw revolution.
Next the screw 9 is moved forward rapidly by a high-speed revolution of the servomotor for injection 3 and the stored melted polymer at the end of the barrel 2 is injected into a cavity 17 as fast as possible and a given pressure is applied for a given duration at the polymer in the cavity 17 and then the injection process is finished and a molding product with a given figure is taken out from the metal mold 1.
It is necessary to get the melted polymer of homogeneous property in the plasticizing process in order to manufacture good-quality molding products. But as the stored melted polymer at the end of the barrel 2 increases in the plasticizing process, an effective length of the screw 9 for plasticizing the resin pellets decreases as the result of the backward movement of the screw 9 in the barrel 2. Therefore, the decrease of the effective length of the screw brings about a variation in the property of the melted polymer, that is, the property of the melted polymer generated at the initial stage of plasticization is different from that of the melted polymer generated at the final stage. To make up for this defect, some methods are applied in which a given pattern of screw back pressure corresponding to the backward movement of the screw 9 is realized in the plasticizing process in order to get a homogeneous property of the melted polymer.
In patent literatures PTL 1 and PTL 2, a given screw revolution is realized by a servomotor for plasticization and the speed control of a screw backward movement by a servomotor for injection realizes a given pattern of screw back pressure.
In patent literatures PTL 3 and PTL 4, a constant speed or a given speed pattern of a screw backward movement is realized by a servomotor for injection and the rotation speed control of a screw by a servomotor for plasticization realizes a given pattern of screw back pressure.
In patent literatures PTL 5 and PTL 6, a given pattern of screw back pressure is realized by a motor current (torque) limit control or a motor current (torque) control of a servomotor for injection.
In patent literatures PTL 7 and PTL 8, the position control of a screw by a servomotor for injection realizes a given pattern of screw back pressure.
In patent literatures PTL 9 and PTL 10, a given revolution speed of a screw is realized by a servomotor for plasticization and a speed control of the screw backward movement by a servomotor for injection realizes a given pattern of screw back pressure and in the speed control of the screw backward movement a set value of screw backward speed modified by a control deviation of the screw back pressure is used.
In patent literature PTL 11, the control mode transfer from the first control mode to the second control mode is carried out. In the first control mode a screw revolution control is carried out by a servomotor for plasticization and a screw back pressure control is carried out by a servomotor for injection. In the second control mode a screw back pressure control is carried out by a servomotor for plasticization and a speed control of screw backward movement is carried out by a servomotor for injection.
In patent literatures PTL 1˜PTL 11, a screw back pressure control is absolutely necessary in the plasticizing process and a pressure detector is required to realize an accurate control of screw back pressure.
In patent literature PTL 12, a pressure detector with a small dynamic range (0˜15.2 MPa) is used for plasticization and a pressure detector with a large dynamic range (15.2˜304 MPa) is used for injection and pressure application. The control accuracy of screw back pressure in the plasticizing process is improved by using a pressure detector with a smaller dynamic range.
FIG. 3 is an explanation drawing which shows a block diagram of an existing plasticizing controller. The plasticizing controller consists of a back pressure controller 20, a motor controller (servoamplifier) for injection 30, a screw revolution speed controller 40, a motor controller (servoamplifier) for plasticization 50 and a pressure detector 12.
The back pressure controller 20 executes a control algorithm at a constant time interval and a discrete-time control is used. The back pressure controller 20 consists of a screw back pressure setting device 21, a subtracter 22, an analog/digital (A/D) converter 23, a pressure controller 24 and a digital/analog (D/A) converter 25. The pressure detector 12 is connected to the A/D converter 23.
The screw back pressure setting device 21 feeds a time sequence of screw back pressure command P*b to the subtracter 22. The pressure detector 12 feeds an actual screw back pressure signal Pb to the subtracter 22 through the A/D converter 23. The subtracter 22 calculates a back pressure control deviation ΔPb=P*b−Pb and the control deviation ΔPb is fed to the pressure controller 24. The pressure controller 24 calculates a motor current demand i*m for the servomotor for injection 3 from ΔPb by using PID (Proportional+Integral+Derivative) control algorithm and feeds the demand i*m to the motor controller for injection 30 through the D/A converter 25.
The motor controller for injection 30 consists of an analog/digital (A/D) converter 31 and a PWM (Pulse Width Modulation) device 32. The motor controller for injection 30 is connected to the servomotor for injection 3 equipped with a rotary encoder 14. The A/D converter 31 feeds the motor current demand i*m from the D/A converter 25 to the PWM device 32. The PWM device 32 applies three-phase voltage to the servomotor for injection 3 so that the servomotor for injection 3 is driven by the motor current i*m. A linear force by the screw 9 applied to the melted polymer stored at the end of the barrel 2 decided by a generated motor current i*m (motor torque) realizes a given screw back pressure P*b.
The screw revolution speed controller 40 consists of a screw revolution speed setting device 41. The screw revolution speed setting device 41 feeds a time sequence of screw revolution speed command N*s to the motor controller for plasticization 50.
The motor controller for plasticization 50 consists of a subtracter 51, a differentiator 52, a speed controller 53 and a PWM device 54. The motor controller for plasticization 50 is connected to the servomotor for plasticization 11 equipped with a rotary encoder 15. The screw revolution speed command N*s from the screw revolution speed controller 40 is fed to the subtracter 51. The rotary encoder 15 mounted on the servomotor for plasticization 11 feeds a pulse train to the differentiator 52. The differentiator 52 detects an actual screw revolution speed Ns and feeds the speed signal Ns to the subtracter 51. The subtracter 51 calculates a screw speed control deviation ΔNs=N*s−Ns and feeds the control deviation ΔNs to the speed controller 53. The speed controller 53 calculates a motor current demand i* for the servomotor for plasticization 11 from ΔNs by using PID control algorithm and feeds the demand i* to the PWM device 54. The PWM device 54 applies three-phase voltage to the servomotor for plasticization 11 so that the servomotor for plasticization 11 is driven by the motor current i* and a given screw revolution speed N*s is realized.
But the usage of the pressure detector in the plasticizing process brings about the following disadvantages.    (1) A highly reliable pressure detector is very expensive under high pressure circumstances.    (2) Mounting a pressure detector in the cavity or the barrel nozzle part necessitates the troublesome works and the working cost becomes considerable.    (3) Mounting a load cell in an injection shafting alignment from a servomotor for injection to a screw complicates the mechanical structure and degrades the mechanical stiffness of the structure.    (4) A load cell which uses strain gauges as a detection device necessitates an electric protection against noise for weak analog signals. Moreover the works for zero-point and span adjustings of a signal amplifier are necessary (patent literature PTL 13).    (5) For the improvement of the control accuracy of screw back pressure, the usage of two kinds of pressure detectors with different dynamic ranges brings about the cost increase (patent literature PTL 12).